This invention relates to the rendering of characters to be displayed on computer output devices. Computer systems typically include some form of visual output device. For example, in FIG. 1, a prior art computer system 10 includes a computer 12, a keyboard 14, a disk storage unit 16, a video display 18, and a printer 20.
Computer 12 includes a central processing unit (CPU) 22, read only memory (ROM) 24, random access memory (RAM) 26, and input/output (I/O) 28. The various components 22-28 of the computer 12 are coupled by a bus system 30. As will be appreciated by those skilled in the art, the representation of the computer 12 has been highly simplified for the purposes of this discussion, but includes the fundamental components of a computer and their interconnections.
Devices such as keyboard 14, disk storage 16, video display 18, and printer 20 are often referred to as “peripheral” devices. When coupled to the computer 12 they become part of computer system 10. Some peripheral devices are primarily for inputting information to the computer 12, other peripheral devices are used for outputting information from the computer 12, and some computer peripherals are used for both inputting and outputting information.
Video display 18 and printer 20 are common output devices for the computer 12. Such output devices typically form temporary or permanent images to convey information to a user of computer system 10. For example, video display 18 includes a screen that can display character, graphical, and other types of images.
A number of problems arise when displaying characters on a computer output device. The term “character”, as used herein, is not limited to alphanumeric characters, but includes any form of character, number, symbol, or other coded device that can be represented or displayed on a computer output device. Some problems relate to the resolution of the output device, and other problems relate to human visual perception. “Resolution”, as it is used herein, refers to the size of individual pixels of a computer output device, and the spacing between the pixels of the output device.
The problems inherent in character imaging tend to be more pronounced for small characters than for larger characters. This is because small characters are made up of fewer pixels and, therefore, even a distortion of a few pixels is readily apparent in the smaller characters. Some of the distortions that may occur, particularly in these smaller characters, are: unbalanced “stem” widths, thin feature disconnection, over/under filling, and inaccurate weighting. One of the most noticeable of these defects is unbalanced stem widths, where vertical and horizontal strokes of characters may be of varying widths due to the character rendering process. Another very noticeable defect is the inaccurate weighting effect of small characters due to a quantization effect. Adding a pixel (the “quantum”) to a character stem that is only a few pixels wide can distort the “weight” given to that character, e.g. it may appear to be “bold” even when it is a normal weight character.
In FIG. 2A, the prior art problem of unbalanced stem widths is illustrated. In this instance, a lower case letter “m” is shown superimposed over a high-resolution grid 32 both in an outline form 34, and a “bitmap” form 36. “Bitmap” refers to a one-to-one mapping between data stored in the memory of the computer 12 and an image of the character to be displayed, i.e. each pixel of the output device corresponds to a bit of information stored in the computer. The outline 34 is typically provided by a parsed font program such as described in Adobe Type I format, Version 1.1, available from Adobe Systems, Inc. of San Jose, Calif., often referred to as the “Black Book” due to the color of its cover. Character outline 34 is converted or “rendered” into the bitmap 36 by a rendering program or “renderer” available from a variety of sources, including Adobe Systems, Inc.
In FIG. 2A, the letter “m” includes some distortion that is attributable to the conversion from font outline 34 to bitmap 36 form. In particular, the three vertical legs or “stems” of the letter “m” are of different widths. As noted, the left stem 38a is two pixels wide, the middle stem 38b is one pixel wide, and the right stem 38c is two pixels wide.
While characters are often rendered on a high-resolution grid as illustrated in FIG. 2A, they must be converted into a “coarse grid” representation before they can be displayed on the output device. The resolution of the coarse grid corresponds to the resolution of the output device.
In FIG. 2B, the conversion from a high-resolution grid to a coarse grid permits the renderer to re-balance the stems. More particularly, the system uses “hints” provided by the parsed font program to stretch and then align the stems of the character to the coarse grid. As noted, in FIG. 2B, the stems 40a, 40b, and 40c are all the same width and, therefore, the unbalanced stem problem can be resolved for black-and-white types of output devices.
While the prior art has solved the problem of unbalanced stem widths for black and white output devices, the problem has re-emerged for grayscale and color output devices. Some output devices use a technique known as “anti-aliasing” in order to provide the illusion of smoother curves and less jagged diagonal lines. This is accomplished by varying grayscale or color values of the coarse grid. However, this manipulation can re-introduce the problem of unbalanced stem widths to anti-aliased characters.
In FIG. 2C, the lower case letter “m” is shown against a coarse grid 42 and corresponding high-resolution grid 44. The high-resolution grid 44 is shown in only one of the cells of the coarse grid 42 so as not to confuse the drawings, although it will be appreciated that the high-resolution grid 44 is associated with all the cells of the coarse grid 42. The coarse grid and the high-resolution grid are related as follows. If the output device has 2n+1 grayscale values, each “cell” 46 of the coarse grid 42 will have 2n pixels 48. To provide a more concrete example, if 17 grayscale levels are provided from pure white to pure black, then 24 or 16 pixels of the high-resolution grid are in each cell of the coarse grid 42. This means that the resolution of the high-resolution grid 44 is four times the resolution of the coarse grid 42. As before, the font outline 50 for the letter “m” is obtained, and a bitmap 52 is developed at the resolution of the high-resolution grid from the font outline 50 by the renderer.
In FIG. 2D, the bitmap 52 has been converted into a grayscale “pixel map” which can be sent to the output device. The conversion is usually accomplished in a straightforward fashion. The number of pixels for a particular cell are counted, and this number is mapped to the grayscale level for that cell. Therefore, cells having more pixels will correspond to a darker grayscale value than cells having fewer pixels. This technique, referred to as “anti-aliasing”, greatly smoothes the appearance of curves and reduces the jaggedness of diagonal lines, as is well known to those skilled in the art.
As used herein, a “pixel map” is much like a “bitmap” except that multiple values are stored in the memory of the computer system 12 which correspond to each pixel of the output device. For example, for each pixel on a grayscale or color video monitor, multiple sub-pixels can be defined each including a numeric value that is stored on the computer.
However, as seen in FIG. 2D, the unbalanced stem problem has reoccurred as a result of the application of the “anti-aliasing” process. More particularly, stem 54a appears to be two pixels wide, while stems 54b and 54c appear to be about one pixel wide. Again, this stem unbalance phenomenon is quite apparent in small characters and reduces the quality of the output image.
While this discussion has centered on vertical stems, that the problem is equally apparent with horizontal stems. For example, while the example of the lower case “m” was given, this stem unbalancing problem is also apparent on letters such as an upper case “E.” Furthermore, both vertical and horizontal “stems” or segments can be found in a single letter, such as the letter “o”, which has two vertical side segments or stems, and two horizontal top and bottom segments or stems.
Some attempt has been made by output device manufacturers to enhance the image provided to the output device as a bitmap or a pixel map. For example, certain Hewlett-Packard Company printers provide “image enhancement” which attempts to smooth “jagged” diagonal lines and to smooth curves. While generally useful, such post-rendering image enhancement mechanisms are not completely successful because the information that was used to create the bitmap or pixel map (e.g. the font outline) is not available to the output device after the render process has been applied. Therefore, post-rendering image enhancement cannot correct problems such as unbalanced stem widths, etc.
Another technique for enhancing the image provided to the output device includes aligning stems on pixel boundaries and is described in co-pending U.S. patent application Ser. No. 08/547,562, filed Oct. 23, 1995, entitled “Method and Apparatus for Rendering Characters,” to Terence Dowling, assigned to Adobe Systems, Inc of San Jose, Calif., the contents of which are expressly incorporated herein by reference. This technique does not suffer from the limitations provided by post-render solutions. However, the technique described in the Dowling application was limited in that the stem alignment was fixed to a single device grid and therefore to the device grid boundaries (pixel boundaries) defined therein.
Certain output devices, such as LCDs support sub-pixel addressing. Sub-pixel, as used herein, refers to a pixel component that is independently addressable in the output device. For example, in a conventional RGB color output LCD device, a single pixel includes three components, or sub-pixels, a red component, a green component and a blue component. Each of the three sub-pixels are independently addressable for each pixel, that is, each individual component can have a value set, the combination of which produce the color perceived by the user for a particular pixel location. In a conventional RGB LCD output device, the sub-pixels include a geometry, the sub-pixel geometry, that defines an physical arrangement of the sub-pixels on the output device. In one conventional RGB LCD output device, each sub-pixel is of the form of a color bar, and the bars are arranged as a horizontal row. Depending on the manufacturer of the output device, the ordering of the color bars may vary from device to device. For example, in a Colorbook device produced by Gateway Computers or a Inspiron 7000 device produced by Dell Computers, the sub-pixel geometry includes an arrangement of color bars in a horizontal row. Other geometries are possible including a closely packed hexagonal arrangement.